Electrostatographic toners used, for example, in electrophotographic dry color copying, are in the form of a finely divided, colored, thermoplastic resin powder. To develop an electrostatic charge pattern or latent image on a charged insulating surface, the toner powder is applied to the surface and the charged toner particles are attracted to and develop the latent image. The developed image is then fixed to that surface or transferred to another surface such as a paper sheet where it is fixed, usually by thermal fusion.
Because of the way in which these colored toners are used, the dyes or colorants contained in them require certain properties. They must, of course, absorb at a desired wave length, or, in other words, they must impart the desired color to the toner. Desirably, they should have high tinctorial strength and should be lightfast. For toners that are to be thermally fixed to a substrate, the dyes must resist decomposition and sublimation at the fusing temperature of the toner. They must also be resistant to migration or so-called "bleeding" from the toner resin into the substrate to which the toner is fused. This quality is especially necessary for toners to be used for developing images on both sides of a paper sheet, i.e., in making two-sided copies. Still further, for multi-color development or in developing color transparencies for optical projection on a screen the toner must be transparent. To form such a transparent image the colorant must be soluble in the thermoplastic binder polymer of the toner. In accordance with the present invention toners are provided which contain dyes having this unusual combination of properties. Certain of the dyes also have other valuable characteristics such as that of not forming stable metal complexes. This is important because the paper that is used as the image support may contain metal ions which form stable complexes with certain yellow dyes. Metallization of the dye can shift its hue from yellow to orange. Preferred dyes of the present invention do not form such stable metal complexes.
A large number of cyano hydroxy pyridone monoazo dyes have been disclosed in the prior art, mostly as textile dyes, e.g., U.S. Pat. No. 4,140,684 and U.S. Pat. No. 4,247,456. Some have been suggested for other purposes, for instance, for coloring polymeric films (U.S. Pat. No. 4,359,418), for use in silver halide photographic materials (U.S. Pat. No. 4,368,260 and U.S. Pat. No. 4,139,383), for use in flowable printing inks (U.S. Pat. No. 4,201,710), or for use as sublimable dyes in thermal transfer recording (Japan Kokai No. 60/27,594). The prior art evidently, however, does not suggest the use of such dyes as colorants for thermoplastic toner compositions. Nor does it suggest the novel dyes of the present invention having the unusual combination of properties required for use in thermoplastic toners which are to be fixed to substrates by thermal fusing.